Hydrocarbon recovery operations may in general involve a broad range of processes involving the use and control of fluid flow operations for the recovery of hydrocarbon from subterranean formations, including for instance the inserting or injection of fluids into subterranean formations such as treatment fluids, consolidation fluids, or hydraulic fracturing fluids, water flooding operations, drilling operations, cleaning operations of flow lines and well bores, and cementing operations in well bores.
Subterranean reservoir formations are porous media comprising a network of pore volumes connected with pore throats of difference diameters and lengths. The dynamics of fluid injection into the reservoirs to displacing the fluids in the porous ground structure in a reservoir has been studied extensively in order to obtain improved hydrocarbon recovery.
The porous ground structure is the solid matrix of the porous media. Elastic waves can propagate in the solid matrix, but not in the fluid since elasticity is a property of solids and not of fluids. The elasticity of solids and the viscosity of fluids are properties that define the difference between solids and fluids. The stresses in elastic solids are proportional to the deformation, whereas the stresses in viscous fluids are proportional to the rate of change of deformation.
The fluids in the reservoir will (during water flooding) experience capillary resistance or push when flowing through pore throats due to the surface tension between the fluids and the wetting condition of walls of the pore throats. The capillary resistance causes a creation of preferred fluid pathways in the porous media (breakthrough), which limits the hydrocarbon recovery considerably. Thus, capillary resistance limits the mobility of the fluids in the reservoir.
The hydrocarbon recovery has been seen to increase after seismic events such as earthquakes. The dramatic dynamic excitation of the formation caused hereby is believed to increase the mobility of the fluid phase in the porous media. It has been claimed that the improved mobility during an earthquakes is caused by elastic waves (in the solid matrix) propagating across the reservoir. Seismic stimulation methods based on inducing elastic waves in the reservoir by applying artificial seismic sources have been investigated. In general artificial seismic sources need to be placed as close as possible to the reservoir to be effective and are thus commonly placed at or near the bottom of the wellbore. Such downhole seismic stimulation tools have been described in e.g. RU 2 171 345, SU 1 710 709, or WO 2008/054256 disclosing different systems where elastic waves in solids are generated by collisions by loads falling anvils secured to the bottom of the well, and thereby on the reservoir formation. Disadvantages of these systems are the risk of fragmentation of the ground structure as well as difficulties in controlling the impact and limited effectiveness of the methods.
Methods for hydrocarbon recovery involving dynamic excitations mimicking seismic events by e.g. use of explosives and regular detonations of energetic materials in the ground have also been developed and extensively used. However, such violent excitations by explosives, earthquakes and the like are often also seen to cause deterioration of the ground structure that may decrease the hydrocarbon recovery over longer time.
Other methods for hydrocarbon recovery involve pressure pulsing by alternate periods of forced withdrawal and/or injection of fluid into the formation. The application of pressure pulses has by some been reported to enhance the flow rates through porous media, but has however also been reported to increase the risk of water breakthrough and viscous fingering in fluid injection operations.
Time dependent pressure phenomenon such as pressure surge or hydraulic shock have primarily been reported on and analysed in relation to their potentially damaging or even catastrophic effects when unintentionally occurring e.g. in pipe systems or in relation to dams or off-shore constructions due to the sea-water slamming or wave breaking on platforms. Water Hammering may often occur when the fluid in motion is forced to stop or suddenly change direction for instance caused by a sudden closure of a valve in a pipe system. In pipe systems Water Hammering may result in problems from noise and vibration to breakage and pipe collapse. Pipe systems are most often equipped with accumulators, bypasses, and shock absorbers or similar in order to avoid Water Hammering.
Another kind of pressure phenomenon (referred herein as impact pressure) is produced by collision processes employing impact dynamics, which makes it possible to generate a time dependent impact pressure with large amplitude and very short time width (duration) comparable to the collision contact time.
In comparison to a pressure wave, pressure pulses can be seen to propagate like a relatively sharp front throughout the fluid. When impact pressure is compared with pressure pulses, one notice that impact pressure has an even sharper front and travels like a shock front. An impact pressure therefore exhibits some of the same important characteristic as pressure pulses, but they possess considerably more of this vital effect of having a sharp front of high pressure amplitude and a short rise time due to the way it is generated. Further, pressure pulses and impact pressure as described in this document are to be distinguished from elastic waves, since these first mentioned pressure phenomena propagate in fluids in contrast to elastic waves propagating in solid materials.